The main application of poly(vinyl butyral) is as interlayers in safety glass laminates. In this application, a pure (colourless, light-stable) product with high hydroxyl content (for good adhesion) is required. Such material is preferably prepared from poly(vinyl alcohol) rather than directly from poly(vinyl acetate). In a typical process, completely hydrolysed poly(vinyl alcohol) is suspended in ethanol and then butyraldehyde and a small quantity of sulphuric acid (catalyst) are added. The mixture is heated at 800℃ for 6 hours. Water is then added to the product with rapid agitation and the poly(vinyl butyral) is precipitated as granules, which are washed with alkali and dried. Poly(vinyl butyral) is generally softer than poly(vinyl formal) with lower softening point, hardness and tensile strength. In commercial polymers the hydroxyl content exerts a major influence on the solubility characteristics. Most grades are soluble in alcohols, glycol ethers and dioxan and as the hydroxyl content decreases the polymers become more soluble in ketones and esters. Poly(vinyl butyral) used in safety glass generally has 78-80% butyral content, 18-19% hydroxyl content and less than 2% acetate content. For this application the polymer is plasticized with an ester such as dibutyl sebacate to the extent of about 30 parts of plasticizer per 100 parts of polymer; the compound is then sheeted by calendering. To produce safety glass, the sheet is placed between two pieces of glass and the laminate is then bonded under heat and pressure.
